The invention relates to a process and a device for continuously drying protein-containing sludge, in particular sewage sludge, in a fluidised bed through which a drying gas flows, wherein partially de-watered sludge is added to the fluidised bed in granulate form and dried sludge is removed therefrom, respectively comprising a drying container having a lower receiving chamber for the drying gas and a gas-permeable support for the fluidised bed, means for feeding the partially de-watered sludge and means for withdrawing the dried sludge, with the feeding means including granulating means.
Sludges of the type in question are present e.g. in purification plants for communal or industrial sewage including at least one biological treatment stage, in paper manufacture or e.g. in the form of oil sludge and as a general rule presenting a solid content of 2 to 5%. By means of mechanical preliminary desiccation, the solid content is mostly raised to 20 to 30%. For subsequent use, e.g. dumping, as an aggregate, as a fuel or as a fertiliser, further drying is required. In most cases storability and sufficient grindability are demanded which are attained only at solid contents exceeding 90% which render the dried sewage sludge biologically stable. Particularly when used as a fertiliser, a grain size varying only within the narrow range corresponding to the one of mineral fertilisers is demanded, such that the dried sludge may be spread in the fields without any modification of available machinery.
It is known from DE 39 02 446 C1 to introduce sewage sludge into an indirectly heated, fluidised sand bed for drying. In this process the sludge is pre-dried such as to comprise a residual water content of approx. 40%. At this water content it is still suited for pumping while, on the other hand, having passed through the so-calledxe2x80x9cstickyxe2x80x9d phase inherently presenting the risk of the material adhering to itself and to the housing even prior to its introduction into the sand bed. It is a drawback of this process that the sewage sludge is altogether comminuted into dust during drying in the sand bed.
From DE 42 42 747 A1, a process for fluidised-bed drying of sludge in the absence of dry foreign matter such as sand is known. The violent frictional motions of particles in the fluidised bed result in abraded particles which are present in the form of fines. In this known process, approx. 90% of the dried sludge are again admixed to the partially de-watered sludge prior to drying. This serves to rapidly pass through the so-called xe2x80x9cstickyxe2x80x9d phase occurring at a content of solids of approx. 40 to 60% during the drying process.
A drawback of this process resides in the low efficiency as only approx. 10%, respectively, of the dried sludge can be withdrawn, and in the high proportion of fines having particle diameters of up to 500 xcexcm, which proportion may constitute up to 20% of the total mass of the dried sludge in this process. Furthermore, in the absence of additional precautions, there is a risk of spontaneous combustion with atmospheric oxygen and of dust explosions owing to the high proportion of fines.
From DE 29 43 558 A1 there is finally known a process for processing sewage sludge, wherein sewage sludge preliminarily de-watered by mechanical means is granulated and dried in a moving-bed dryer. For granulation of the sludge preliminarily de-watered by mechanical means, i.a. a dried granulate is admixed in the mixing granulator. The mixed granulate thus produced already includes a relatively high content of dry substances and has therefore, upon entering into the fluidised bed, already left the xe2x80x9cstickyxe2x80x9d phase. By providing the sludge in granulate form, i.e. in the form of a multiplicity of granules, a large particle surface and thus good heat transfer from the drying gas is achieved. This fundamentally allows for efficient utilisation of the input energy.
The efficiency of this process is, however, relatively low owing to recycling or admixture, respectively, of dry substances.
It is an object of the invention to furnish a process and a device of the type indicated in the introduction for drying protein-containing sludge, in particular sewage sludge, whereby the addition of foreign matter in the fluidised bed as well as recycling of already dried sludge and dust into the process may be avoided, resulting in increased efficiency and lower energy consumption.
In terms of process technology, this object is attained by forming the granules without the addition of dried substances, and through the circumstance that the granulating process is preferably combined with pressing; in terms of device technology it is achieved in that means for the admixture of dried substances are eliminated in the feeding means.
As a result, surprising phenomena may be utilised in the fluidised bed for drying sludge. Thus it was found that the granulate shape enables admission of the sticky phase into the drying container.
In this continuous process the fluidised bed contains granular particles throughout the various stages of drying. This allows for omitting the recycling of dried material inasmuch as a sufficient quantity of dried granular material for rapidly passing through the sticky phase is present in the drying container at all times. Due to mixing with the dried granular material, accelerated surface drying to a content of solids of more than 60% is achieved, such that the granular material is subject to the risk of adhering for only a short period.
It is another surprising advantage that a major proportion of the dust formed as a result of abrasion of already dried granular material is bound to the humid particles in the fluidised bed which are still in the sticky phase. This reduces the quantity of dust and efficiently prevents deposition of particles in the sticky phase on plant components as well as agglomeration thereof. This is particularly true if the drying gas is circulated without dust recovery, with dust recovery being performed only on the exhaust vapor exiting from the dryer.
Moreover the pressing of the granular materials concurrently performed with shaping by means of granulating means provided in the function of a granulator is of advantage. Hereby the granular material is given an initial stability which is advantageous for drying in the fluidised bed. There would otherwise exist the danger of the particles disintegrating in the fluidised bed, and further use of the granular materials would not be possible. The newly supplied particles are mixed by the inflow of drying gas with the already dried granules in the fluidised bed, and due to the resulting thorough mixing, the large particle surface and selection of a suitable drying gas, rapid drying or curing on the particle surface is achieved, resulting in the granular material maintaining a stable shape and the granules hardly adhering to each other.
As the granular material supplied to the fluidised bed includes a high water content of generally about 75%, the particles shrink during drying. Owing to the irregular shrinking engendered by rapid surface drying while the interior is still humid and owing to the various degrees of abrasion, an irregularly shaped dry granulate including cavities in which dust accumulates during drying is produced in the course of the drying process.
This considerably reduces the quantity of dust while precluding the risk of dust explosion in the absence of any additional measures.
Size and shape of the dried granular material may be influenced by using various types of granulators and by varying the operating parameters during grain conformation. Hereby a narrow grain size distribution at grain diameters of a few millimeters and adaptation to the specific application and to customers"" specifications are achieved, e.g. for use in fertilizer spreaders or for pressurized air injection in combustion facilities. The granular material has granules which are preferably 1-10 mm in average diameter, more preferably 3 to 7 mm and most preferably about 5 mm.
Where the granular material is supplied onto the fluidised bed by corresponding technical devices immediately following its conformation and by gravity, clotting of the grains and adhering to plant components may be avoided and good mixing with the particles already dried on the surface may be achieved.
Directly supplying the granular material below the surface of the fluidised bed ensures that the granular material is exposed to the drying process without delay.
Where a preliminary product for the granular material is used, e.g. in the form of notched rods, this allows for simplification of the shaping process and of the granulating means without impairing the quality of the final product.
By limiting the supplied granules to a particular range of diameters, i.e., realisation of a specific particle shape and size, the respective users"" demands with respect to storability, transportability and applicability of the final product can be met.
Drying of the sludge to contents of solids of more than 90% guarantees biological stability of the final product.
Owing to the preferred use of superheated water vapor as the drying medium, the high specific thermal capacity thereof is made use of and very good heat transfer to the material to be dried is achieved. As a result, the sticky phase is passed through rapidly, energy input is utilised efficiently, and a lower energy consumption is achieved.
The expelled exhaust vapor may be condensed practically entirely, and the condensate may be supplied to the purification plant. As long as the water vapor temperature does not exceed a value of approx. 150xc2x0 C., decomposition of the organic matter of the sewage sludge hardly takes place. The expelled exhaust vapor contains only small amounts of non-condensable gases in this case, so that only small amounts of exhaust gas have to be purified, or exhaust gas purification will be unnecessary under favorable circumstances.
The superheated water vapor moreover does not contain any oxygen and thus inherently precludes any danger of spontaneous combustion of the material to be dried and of dust explosion in the container.
As the temperature of the material to be dried is higher than the boiling temperature of water during the drying process, an environment in which any pathogenic germs possibly remaining in the sludge are destroyed is created in the effective range of the water vapor.
The process is started up by using an already dried granular material because a sludge only partially de-watered might adhere to itself and to the plant components and thus hamper build-up of a fluidised bed.
Utilisation of heat exchangers comprising respective heat exchanger surfaces in the fluidised bed permits efficient input of heat energy. This constitutes a considerable advantage as the superheated water vapor already transmits a considerable amount of energy to the granular material over the initial few centimeters of the fluidised bed.
Where saturated steam at preferably 5 to 25 bar pressure above atmospheric is used as a heating medium for the heat exchangers, transfer of a great amount of energy may be achieved at a relatively low throughput of heating medium.
A slight pressure above atmospheric in the drying plant prevents the intrusion of air through leaks.
A slight vacuum, on the other hand, prevents unpurified exhaust vapor from escaping from the drying plant.
It is another advantage of the process that owing to compression of the exhaust vapor expelled from the sludge by means of a corresponding compressor means and subsequent condensation at an elevated temperature in a heat exchanger, such as preferably the heat exchanger in the fluidised bed, evaporation energy of the expelled water may be recuperated as condensation heat and recycled into the drying process. This further reduces energy consumption.
An advantageous embodiment of the device may be realised by omitting admixing means for dry substances. Thus the expense in terms of device technology may be reduced and control of the drying plant is simplified substantially.
Positioning the outlet range of the granulating means above the fluidised bed is advantageous inasmuch as the outlet opening is not affected by impinging particles in this case.
Positioning the outlet range of the granulating means within the fluidised bed has the result that the granular material is immediately exposed to the effect of the fluidised bed. Particles possibly adhered in the outlet range of the granulator will again detach concurrently with ongoing drying under the influence of already largely dried particles of the fluidised bed.
It is another advantage if a granulating means is used wherein shaping is combined with pressing. This serves to reduce the risk of agglomeration of the granulate in the granulator or during transport into the dryer, and in addition less external air is admitted into the dryer. Moreover the granular material upon its entrance into the fluidised bed already has essentially better dimensional stability than in the case of a granulating process without pressing, such as e.g. by rotating blades.
Where the drying container is designed to be pressure-tight, air is prevented from entering and unpurified exhaust vapor is prevented from escaping.